Motorways leading from coastal areas were jammed as authorities issued urgent warnings to hundreds of thousands of residents to flee. Some motorways were to be turned into one-way roads to speed the exodus from the storm zone.

There were also reports of massive power outages as the storm approached Texas' coast.

Harvey, the first major storm of the annual Atlantic hurricane season, was packing maximum sustained winds of 195km per hour.

The storm is expected to dump up to 89cm of rain over a four or five-day period in parts of Texas.

Major test for Trump

Satellite images showed the massive storm system extending hundreds of kilometres into the Gulf of Mexico.

It is forecast to be the most powerful hurricane to hit the mainland since Wilma struck Florida in 2005, and could inflict billions of dollars in damage.

Before Wilma, Hurricane Katrina pummelled New Orleans in the same year, leaving more than 1,800 people dead and becoming a major failure of the presidency of George W Bush.

'Just terrifying'

In 2005, Bush faced severe criticism after FEMA appeared unprepared for the devastating damage inflicted by Hurricane Katrina.

"Keep on top of hurricane Harvey don't make same mistake President Bush made with Katrina," Republican Senator Chuck Grassley urged the Trump in a tweet.

FEMA chief Brock Long said the most pressing danger was the storm surge, the high tides powered by powerful winds - expected to reach between 1.8 metres and 3.6 metres in some areas - but said many inland counties should prepare for "significant" flooding.